1. Field of the Invention
The present invention is directed to the field of circulating molten metals in a molten metal bath, and specifically to melting and circulating metal scrap being added to a molten metal bath. Due to the high temperatures required for melting metal scrap, the impeller is intended for extremely high heat applications.
2. Description of the Prior Art
Limitations in natural resources, and concerns for reducing environmental pollution, have caused significant growth in the metal recycling industry, also known as the secondary metal industry. The secondary metal industry collects metal scrap, such as aluminum, and remelts the scrap metal to create usable recycled metal. During the remelting process, oxides and gases are generated which can become entrained in the molten metal bath, and render some of the recycled metal as unusable. In order to minimize the effect of these oxides or gases, a flux is added to the surface of the bath, which causes impurities in the molten metal to rise to the surface, and create a crust or "dross", on the surface of the bath. A more complete discussion on the formation and treatment of this "dross" exists in U.S. Pat. No. 4,598,899, which is hereby incorporated by reference.
It is known that the utilization of molten metal pumps is extremely helpful in circulating the molten metal within the bath, to ensure that the temperature of the bath remains constant throughout, and to ensure that unmelted pieces of metal are circulated into the bath, so that they melt and mix therewithin.
Due to the high surface tension which is characteristic of molten metals, it is somewhat difficult to force lighter gauge metal scrap to circulate and melt within the metals. On the contrary, heavier scrap sinks to the bottom where it melts due to the high temperature therein. Therefore, applications where lighter metal scrap are melted are in need of these types of devices. Due to the highly corrosive and destructive environment that such impellers are intended to be used, these devices are typically made from graphite or other refractory materials which are sufficient to resist the harmful effects of this environment.
U.S. Pat. No. 4,598,899, discloses one type of melter for this application. This device utilizes a circular blade or auger made up of a plurality of blades formed as a spiral flute, wherein the auger forms a cylindrical shape. A cylindrical auger drum is attached to the outside of the auger, and cemented thereto, forming an auger assembly. The auger assembly is attached to the bottom of a rotating shaft, and projects into a molten metal bath. The shaft is rotated by an external motor, and acts to pull the molten metal down from a top side of the auger and push it downward; a secondary circulating pump is used to direct the output of the auger and form an overall internal convection path between baffles in the molten metal bath.
Certain shortcomings exist in this design. Because of the need for a two-piece auger assembly, manufacturing and assembling costs are significantly increased. Additionally, due to the thin spiral shape of the blades, damage and deterioration can occur as a result of the blades hitting unmelted metal. A further disadvantage is that the auger drum assembly can become clogged with debris, reducing or eliminating efficiency.
U.S. Pat. No. 4,898,367, which is hereby incorporated by reference, discloses a method of dispersing gas into molten metal, wherein a graphite shaft has an "impeller" on the end. The shaft is hollow, such that gas can be injected through the tube, and into the metal bath. The impeller at the end of the shaft is a simple square shape; as the shaft rotates, large bubbles exiting through the discharge opening in the bottle of the shaft will flow outwardly along the bottom face of the impeller, and contact the sharp corners of the rectangular impeller. The bubbles are therefore sheared into finally divided bubbles, and thrown outwardly and mixed with the molten metal. There is no teaching, however, of the impeller having any shape which would result in a significant movement or circulation of the molten metal.